Today, functional optical devices integrated in a silicon (Si) photonic substrate attract attention. Such devices are fabricated making use of conventional and inexpensive technologies of forming microelectronic circuits, enabling large-scale integration on Si substrates. For high-performance servers or supercomputers, computing performances have been improved by employing multi-core CPU architectures, responding to demands for higher computation abilities. However, electrical data transmission between chips or boards is facing limits due to physical distances and it becomes difficult to catch up with high-speed computation. Silicon (Si) photonics technology is expected as an alternative technology that solves the problem of insufficient electric transmission capacity between high-rate data processors. Si photonics technology enables integration of Si-waveguide-based optical devices on a Si substrate. Especially, applications of wavelength division multiplexing (WDM), that has already been put into practical use in the telecommunications field, to Si photonics is expected as key technology for high-density transmission and reduction of fiber optical cable.
For Si photonics-based optical transmission equipment such as optical transmitters, optical receivers, optical switches, or optical routers, compound III-V semiconductors including GaAs-based or InP-based materials have been used in light sources, optical amplifiers, loss-compensation optical devices and so on because bulk Si has an indirect bandgap. It is difficult to monolithically integrate Si-waveguide-based devices and III-V semiconductor components on the same substrate due to lattice mismatch. Currently, hybrid integration to place compound semiconductor optical components onto a Si waveguide platform is a mainstream.
In hybrid integration, a structure for optically coupling the optical waveguide of a III-V compound semiconductor chip to a Si photonic waveguide formed on a Si platform by abutting connection at end faces is known. See, for example, Japanese Patent Application Laid-open Publication No. 2007-286340 (Patent Document 1). Another known structure is to provide evanescent coupling by placing a gain medium of III-V compound semiconductor in close proximity to a Si photonic waveguide on a Si platform. See, for example, S. Stankovic et al., “Hybrid III-V/Si Distributed-Feedback Laser Based on Adhesive Bonding”, IEEE Photonics Tech. Lett., Vol. 24, No. 23, Dec. 1, 2012 (Non-patent Document 1). Evanescent-coupled devices cannot achieve sufficient characteristics because of large optical loss due to incompleteness of binding interfaces. As a modification of end-face coupling structures, a technique of mounting semiconductor optical amplifiers (SOAs) by flip-chip bonding onto solder bumps formed over a Si waveguide platform is proposed. See, for example, R. A. Budd et al., “Semiconductor Optical Amplifier (SOA) Packaging for Scalable and Gain-Integrated Si photonic Switching Platforms”, 2015, Electronic Components & Technology Conference (Non-patent Document 2).
When assembling a semiconductor photonic chip on a Si photonic platform (which may be called simply as “Si platform”), it is desired to couple the optical waveguide of the semiconductor photonic chip to the Si photonic waveguide of the Si platform at low coupling loss. With an end-face coupling structure, the end face of the optical waveguide of the semiconductor photonic chip has to be brought into the right position so as to face and align with the end face of the Si photonic waveguide formed on the Si platform to provide sufficient optical coupling between them. When mounting a SOA chip on a Si platform by end-face coupling, typically the SOA chip is embedded in a recess opening formed on the Si platform, and the optical waveguides of the SOA chip and the Si photonic waveguides on the Si platform are optically connected to each other in a horizontal direction parallel to the Si platform surface. With this scheme, the alignment accuracies in the length, the width, and the height directions between two waveguides become factors for determining optical coupling loss. In order to achieve high optical gain in a SOA with input and output waveguides, low-loss optical coupling is demanded at both the input waveguide and the output waveguide. However, it is difficult to achieve high optical gain for several reasons described below.
An optical circuit structure in which a semiconductor photonic device is integrated with low coupling loss is desired.